Dry powder mixes comprising phase change materials

ABSTRACT

Free flowing, conformable powder-like mix of silica particles and a phase change material (p.c.m.) is disclosed. The silica particles have a critical size of about 7x10-3 to about 7x10-2 microns and the pcm must be added to the silica in an amount of 80 wt. % or less pcm per combined weight of silica and pcm. The powder-like mix can be used in tableware items, medical wraps, tree wraps, garments, quilts and blankets, and in cementitious compositions of the type in which it is beneficial to use a pcm material. The silica-pcm mix can also be admixed with soil to provide a soil warming effect and placed about a tree, flower, or shrub.

GOVERNMENT RIGHTS

The Government has rights in this invention pursuant to Contract No.DE-FG03-86SF16308 awarded by the U.S. Department of Energy.

RELATED APPLICATIONS

This application is a continuation-in-part of Ser. No. 319,835, filedMar. 3, 1989, which, in turn, was a continuation of Ser. No. 096,288,filed Sept. 11, 1987, now abandoned, which, in turn, was acontinuation-in-part of Ser. No. 937,866, filed Dec. 2, 1986, nowabandoned, which, in turn, was a continuation-in-part of Ser. No.801,127, filed Nov. 22, 1985, now U.S. Pat. No. 4,711,813 (Salyer).

FIELD OF THE INVENTION

This invention relates to a dry, freely flowing powder mix comprising aphase change material.

BACKGROUND OF THE INVENTION

Phase change materials may be repeatedly converted between solid andliquid phases and utilize their latent heat of fusion to absorb, storeand release heat or cool during such phase conversions.

These latent heats of fusion are greater than the sensible heatcapacities of the materials. For example, in phase change materials, theamount of energy absorbed upon melting or released upon freezing is muchgreater than the amount of energy absorbed or released upon increasingor decreasing the temperature of the material at an increment of 1° C.

Upon melting and freezing, per unit weight, a phase change material(P.C.M.) absorbs and releases substantially more energy than a sensibleheat storage material that is heated or cooled to the same temperaturerange. In contrast to a sensible heat storage material that absorbs andreleases energy essentially uniformly over a broad temperature range, aphase change material absorbs and releases a large quantity of energy inthe vicinity of its melting/freezing point.

Phase change materials capable of storing and releasing thermal energyhave found many applications in building structures, road basematerials, beverage and food containers, medical wraps, and textileapplications such as garments. One of the basic problems, however, inthe use of solid-to-liquid PCMs for control of temperature, iscontainment. That is, for heat transfer efficiency as well as safetypurposes, it is undesirable to have a thick block or agglomeration ofsolid phase PCM below the PCM melting point. Similarly, when above themelting point, PCM in liquid phase can be problematic. For instance,building panels containing liquid phase PCM have proven deficient. Inone such PCM-containing panel, carpenters reported that a liquid phasePCM leaked out of the panel when nails were driven through it.

In those situations in which medical hot or cold packs containing PCMsare used, a solid phase agglomerate of PCM below its melting pointrenders the structure unwieldy and incapable of conforming about therequired body part to achieve the desired heating or cooling function.

Accordingly, it is an object of the invention to provide a conformable,powder-like PCM-matrix composite that will not liquefy upon heating ofthe PCM above its melting point and will not form a rigid solid attemperatures below the melting point. In other words, it is desirable tofind a new method of containment for the PCM wherein, when above orbelow its melting point, the PCM-matrix structure will be in the form ofa soft, conformable configuration like a sand pack.

SUMMARY OF THE INVENTION

I have found that a very small size silica filler may be used as amatrix for the PCM. This silica filler has particle sizes on the orderof about 0.007 to 0.07 microns in diameter and is capable of absorbingfive to ten times its weight of liquid PCM. The silica filler isliterally stirred into the liquid PCM at a temperature that is above themelting point of the PCM. At combinations of PCM/silica filler of90/10-85/15 (weight) a gel composition is obtained. However, when mixedat 80/20 PCM/silica filler and at lower PCM content, a free-flowingpowder is obtained that remains free flowing above and below the meltingtemperature of the PCM. This type of structure is especially desirablefor hot and cold medical wrap applications, but is of interest in otherapplications such as for citrus tree wraps, tableware, buildingstructures, soil admixtures, garments, blankets, quilts, etc.

In those situations in which the PCM/silica mix is to be used as a hotmedical wrap, it is desirable to provide a microwavable packagecontaining the mix. To enhance this capacity, polar additives may beadded to the mix to absorb microwave energy effectively or the PCMitself maybe a polar compound such as a high molecular weight (i.e.,≧1,000) polyethyleneglycol material.

A variety of different PCM materials may be used in the silica-PCMmixture as long as the melting and freezing temperatures thereof fallwithin a broad range of between about -20° to about 140° C. The lowermelting PCMs are useful for medical therapy cold pack, citrus tree frostprotection and soil admixtures, with the higher melting PCMs beinguseful for medical therapy heat packs, tableware, etc. It is preferredthat the PCM have a latent heat of fusion of about 30 cal./gram orhigher.

PRIOR ART

The broad idea of using silica as a suspension medium for PCMs inbuilding blocks is not new. For instance, see U.S. Pat. No. 4,259,401(Charoudi et al) wherein this concept is disclosed at column 21, line 60et seq.

A microwavable heat storage food container comprising wet sand as athermal storage medium is disclosed in U.S. Pat. No. 4,567,877(Sepahpur). Of further possible interest is U.S. Pat. No. 4,367,788(Cordon) which is directed toward the use of PCM materials such asglauber's salt that are absorbed by porous perlite materials. Otherpatents of possible interest are U.S. Pat. No. 4,294,078 (MacCracken)and U.S. Pat. No. 4,711,813 (Salyer).

The prior art however does not suggest utilization of the combination ofspecific particle size silica and PCM/silica weight ratios hereinrequired in order to result in a dry, conformable, powder-like, PCMcontaining composition that may be useful in widespread environments.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with the invention, a dry powder-like PCM-silica mix isprovided that may be used for: medical hot and cold pack applications,as a wrap for citrus trees, or in admixture with soil to protect treesor plants implanted therein, building structure applications such as inplasterboard, and food and tableware accessory items, etc.

The particle size of the silica is critical. I have found thatsub-micron, ultra fine particle size silicas on the order of about7×10⁻³ to about 7×10⁻² microns (in diameter) can be manually mixed in asolution of phase change material within a critical addition range. Thatis, the ultra-fine silica should be added to the phase change materialan amount of about 80% PCM (weight PCM based on total weight of PCM andsilica in mix) or less. If more than about 80% of the PCM is added, agel-like mixture is provided. However, at about 80/20 PCM:silica, afree-flowing powder is obtained that remains free-flowing both above andbelow the melting temperature of the PCM. This type of structure isespecially desirable for hot and cold medical wraps, but is of interestfor other applications as well (such as in citrus tree wraps, tableware,garments, blankets).

The preferred addition range for the PCM is from about 80%-50% (byweight based upon total weight of the composite, i.e., silica-PCMmixture). For convenience in mixing, I have usually added the dry silicato liquid PCM (i.e., PCM maintained at a temperature higher than itsmelting point). However, the reverse addition of PCM to silica can alsobe accomplished with suitable equipment to prevent aerosolization of thefinely divided silica.

As to the silica materials which may be used, these are sub-micron sizesilicas having particle sizes on the order of 0.007 to about 0.07microns. The finely divided silicas that have been used successfully todate have come from commercially available pigmentary grades of silicasuch as are used as reinforcing filler in rubbers. These pigmentarygrades of silica are orders of magnitude smaller in size than sandparticles and have other important differences as well.

Typically, these sub-micron silicas are made by one of two differentprocesses. For example, fumed silica is made by hydrolysis of silicontetrachloride vapor in a hydrogen/oxygen flame. Precipitated silicas aremade from an alkaline silicate (e.g., sodium silicate) that isprecipitated with a mineral acid or metal salt.

Both procedures produce a small spherical particle with multiplehydroxyl groups located at the surface and a three-dimensionalchain-like structure (as in carbon black). The chain structure isthought to be very important to the reinforcing, gel formation, andother properties of the silicas. The surface area of the silicas mayrange from 50 square meters/gram to 500 m² or even higher.

Exemplary silicas include the "Cab-o-Sil®" series of fumed silicasavailable from Cabot Corporation, Tuscola, Ill., and the "Aerosil®", FKseries, Sipernat® series, Ultrasil® and Quso® series silicas availablefrom DeGussa. Hi Sil® precipitated silicas from PPG may also bementioned as being useful. At present, the preferred silica isCab-o-Sil® MS-7SD fumed silica from Cabot. This particular silica hasthe following physical characteristics.

    ______________________________________                                        Surface Area (m.sup.2 /g)                                                                         175-225                                                   pH (4% Aqueous Dispersion)                                                                        3.7-4.3                                                   Density (lbs./cu. ft.)                                                                             9-11                                                     Wt. % Moisture       1.5 max                                                  Silica content      99.8 min                                                  Specific Gravity    2.2                                                       Refractive Index    1.46                                                      Color               White                                                     X-Ray Form          Amorphous                                                 ______________________________________                                    

As to the PCMs that can be used, crystalline alkyl hydrocarbons having achain length of C₁₄ and greater are preferred for most situations. OtherPCMs that may be mentioned include water, glycerine, crystalline fattyacids, crystalline fatty acid esters, crystalline alicyclichydrocarbons, crystalline aromatic compounds, hydrated salts (Glauber'sSalt preferred), the clathrates, semi-clathrates, gas clathrates,polyethylene glycol, and halogen-terminated alkyl hydrocarbons.

Suitable clathrates include those which consist of either a noble gas(i.e., the gas clathrates) or a non-polar halocarbon which formshydrates in as little as 10% concentration. The chlorofluorocarbonclathrates tend to be relatively expensive and are, therefore, notpreferred. Additionally, some specific chlorofluorocarbons (e.g., Freon11, 12, etc.) are also suspected to contribute to depletion of theearth's ozone shield and are undersirable for this reason as well.

Promising clathrate pcms also include the quaternary amine salts withhalogen or other acids (clathrates or semi-clathrates). These hydratesare pseudo compounds, wherein the crystals of "ice" are able to hostorganic molecules (of specific composition) in nearly spherical cages oftwo different sizes. If only the larger of the two cages is occupied bythe guest molecules, the pcm may contain 33 or more molecules of water.If both cages are occupied by guest molecules, the PCM will containabout 17 molecules of water. In either case, the water content in theseclathrate and semi-clathrate pcms is much higher than in some of thesalt hydrates such as sodium sulfate decahydrate.

Nearly all hydrated salts can be employed, with various degree ofsuitability, as PCM. The only such materials which are wholly unsuitableare those which decompose, rather than melt. Marginally suitablehydrated salts are those which melt incongruously, those with low heatsof fusion, and those with melting points which lie outside (generallyfar above) desired temperature ranges. Nevertheless, there are a widevariety of meltable hydrated salts with high heat of fusion and useablemelting points; and many of these satisfy stringent cost requirements.The preferred hydrated salts are those which are formed primarily fromthe combination of positive ions of sodium, potassium, calcium, ammoniumand iron with negative ions of acetate, silicate, chloride, nitrate,mono, di, and tri basic phosphate, mono and di basic carbonate and monoand di basic sulphate. Other ions may be added to the above combinationsin small quantities, (although they are more expensive) in order toadjust melting point or to obtain other desired properties. Virtuallyall such combinations will function in the desired manner; and most havemelting points in the useful range, for example: Fe₂ O₃.4SO₃.9H₂ O,NaNH₄ SO₄.2H₂ O, NaNH₄ HPO₄.4H₂ O, FeCl₃.2H₂ O, Na₃ PO₄.12H₂ O, Na₂SiO₃.5H₂ O, Ca(NO₃)₂.3H₂ O, K₂ HPO₄.3H₂ O, Na₂ SiO₃.9H₂ O, Fe(NO₃)₃.9H₂O, K₃ PO₄.7H₂ O, NaHPO₄.12H₂ O, CaCl₂.6H₂ O and Na₂ SO₄.10H₂ O, Na(CH₃COO).3H₂ O. The specific melting point desired is obtained by varyingthe degree of hydration and by alloying it to form binary or trinaryeutectics.

As above noted, the crystalline alkyl hydrocarbons having a carbon chainof about 14° C. atoms or greater are preferred. These waxes arecommercially available under a host of trademarks. For instance, thesecommercially available waxes include: Shellwax® 100 (MP 42°-44° C.),Shellwax® 120 (MP 44°-47° C.), Shellwax® (MP 52°-55° C.), Shellwax®300(MP 60°-65° C.) all of which are products of Shell Oil Co.; Boron R-152(MP 65° C.) a product of Standard Oil of Ohio (SOHIO); Union SR-143 (MPabout 61° C.) a product of Union Oil Co.; Witco 128 (MP about 53° C.)Witco LLN, Witco 45A, Witco K-61, Witco K-51, and Witco 85010-1 allproducts of Witco Corporation (Kendall Division); Aristowax®143 (MP34°-61° C.), and Paraffin 150 (MP about 61° C.). These waxes have heatsof fusion greater than 30 cal/g and by comparison to other phase changematerials, they are inexpensive.

One group of waxes for use in the present invention includescommercially available mixtures of crystalline alkyl hydrocarbons. Thesemixtures of alkyl hydrocarbons are obtained at low cost as by-productsof petroleum refining. Typically, these are blends of alkyl hydrocarbonswhich differ by no more than 4 or 5 carbon atoms. A typical example isWitco 45A which contains about 21% C-18, 33% C-19, 26% C-20; 11% C-21hydrocarbon, and the balance higher and lower hydrocarbons. Because theyare inexpensive, they can be incorporated into the silica-pcm compositeat minimal additional expense and, at the same time, provide highsavings in terms of reduced energy costs.

While these waxes are mixtures they exhibit one melting freezing pointwhich is the average of the melting freezing points of the constituents.Some blends for passive heating and cooling have a melting and freezingpoint in the range of 24° to 33° C. Some blends for passive cool storagehave a melting and a freezing point in the range of 0° to 33° C. In manyapplications, the blends will be relied upon for both heating andcooling and will be characterized by having both the melting and afreezing point in the range of 20° to 25° C.

Ultra pure alkyl hydrocarbons C-14 to C-22 and higher are also availableat a premium cost. These may have higher heats of fusion andcrystallization (e.g., 55-60 cal/g) than the low-cost mixtures describedabove. These ultra pure alkyl hydrocarbons are also useful in thepresent invention for critical applications requiring maximum storagecapacity in the minimum volume of space.

Another consideration in the selection of waxes used in the presentinvention is the difference between the melting and freezing points. Thealkyl hydrocarbons are self-nucleating and thus melt and freezecongruently. Thus, when heated or cooled at rates of 2 C./min. or less,the melting and freezing temperatures substantially coincide.

Additionally, the halogen terminated alkyl hydrocarbons can be useful asa pcm and also provide fire retardancy.

When the powder-like silica/pcm mixture is to be used for medical hotwrap applications, it is desirable to heat the mix via microwave orother dielectric heating means. In such cases, a polar pcm such aswater, glycerine, ethylene glycol, a high molecular weight (i.e.,greater than 1,000) polyethylene glycol, the clathrates,semi-clathrates, gas-clathrates and hydrates may be used.

Alternatively, if a microwavable product is desired, a non-polar pcmsuch as the crystalline long chain (C₁₄ and greater) alkylhydrocarbons,crystalline fatty acids, crystalline fatty acid esters, crystallinealicyclic hydrocarbons and crystalline aromatic hydrocarbons can be usedprovided they are used conjointly with polar compounds such as water,ethylene glycol, glycerine, polyethylene glycol, and ivory liquid, etc.Such polar compounds should be added to the silica-pcm composite in anamount of from 5-25 wt. %, preferably 5-10 wt. % (based upon the totalweight of the silica/pcm/polar compound combination).

In those end use environments in which high humidity conditions areencountered, the silicas tend to have a greater affinity for water thanfor the preferred alkyl hydrocarbon PCM. This limitation is important inattempting to add the silica-liquid alkyl hydrocarbon (C₁₄ and greater)to a wet mix of plaster or concrete. In these cases, preliminary testshave demonstrated a partial separation of the PCM from the hydrophilicsilica that comprises a multiplicity of surface hydroxyl groups.However, to minimize such phase separation problems, surfacemodification of the hydrophilic silicas to form hydrophobic silicaappears promising. For example, the fumed or precipitated ultra-finesilicas are normally hydrophilic. However, when modified by treatmentwith a suitable coupling agent, for instance, dimethyldichlorosilane,divinyldichlorosilane, hexamethyldisilazane, etc., a hydrophobic silicahaving different properties from the untreated base silica is formed.Although applicant is not to be bound to any particular theory ofoperation, it is thought that the treatment with the coupling agentreplaces many of the available surface OH groups on the silica particleswith organic moieties such as alkyl or aryl groups. This renders thesilica hydrophobic.

One such "surface modified" silica is sold by Cabot under the"Cab-o-Sil® 610" trademark. This hydrophobic silica is therefore highlypreferred in those situations in which the silica-pcm powder mix will besubjected to high humidity or aqueous environments such as those thatmay be found when the mix is to be used in building structures, forexample, in cementitious mixes such as in plaster, plasterboard andconcrete mixes. Further, the hydrophobic silicas would be used in thosesituations in which the silica-pcm powder mix is to be admixed with soilso as to provide controlled heat release to plants or trees implanted inthe soil.

"Cab-o-Sil® 610" has the following physical characteristics:

    ______________________________________                                        Appearance       Fluffy white powder                                          surface area (BET) (m.sup.2 /g)                                                                120 ± 15                                                  pH.sup.1         4.0-5.0                                                      Carbon (wt. %)   0.85 ± 0.15                                               Bulk Density (lbs./cu. ft.)                                                                    ≈3.0                                                 Loss on Heating.sup.2 (wt. %)                                                                  0.5                                                          ______________________________________                                         .sup.1 4 g in 100 g of 80/20 isopropanol/water by weight                      .sup.2 2 hours at 110° C.                                         

The surface structure of "Cab-o-Sil® 610" is thought to be as follows:##STR1##

Another exemplary hydrophobic silica is available from Cabot under the"Cab-o-Sil" 720 trademark. This hydrophobic silica has had its surfacemodified via reaction with hexamethyldisilazane and is thought to havethe surface structure of: ##STR2##

Other exemplary hydrophobic silicas include Aerosil® R 972 and Aerosil®R 974, available from Degussa.

As used herein in the specification and claims, hydrophobic silica isused to refer to a silica that will not exhibit separation of pcm fromthe pcm-silica powder mix when the mix is subjected to water or highhumidity conditions (i.e., ≧90% R.H.).

DRAWINGS

The invention will now be further explained in conjunction with theattached drawings. In the drawings:

FIG. 1 is a diagrammatic view of a medical wrap utilizing the silica-pcmpowder-like composite of the invention;

FIG. 2 is a sectional view taken along the lines and arrows 2--2 shownin FIG. 1;

FIG. 2A is a magnified cut-away view showing the single phase nature ofthe silica/pcm combination;

FIG. 3 is a diagrammatic view of a citrus tree wrap incorporating thesilica-pcm powder-like composite of the invention;

FIG. 4 is a diagrammatic view of a tableware item, a dinner servingtray, utilizing the silica-pcm powder-like composite of the invention;

FIG. 5 is a sectional view taken along the lines and arrows 5--5 in FIG.4.

FIG. 6 is a diagrammatic view of a garment, a vest, that incorporatesthe silica-pcm powder-like composite of the present invention;

FIG. 7 is a cross-sectional view taken along the lines and arrows 7--7shown in FIG. 6;

FIG. 8 is a diagrammatic view of a quilt incorporating the silica-pcmpowder-like composite of the invention; and

FIG. 9 is a cross-sectional view taken along the lines and arrows 9--9shown in FIG. 8.

Turning now to the drawings and to FIGS. 1 and 2 thereof, there is shownmedical wrap 2, specifically a knee joint wrap comprising outer envelope4, formed from a liquid impervious polymer such as abutadiene-acrylonitrile copolymer, a polyester such as polyethyleneterephthalate or vinyl polymer such as plasticized polyvinyl chloride,plasticized polyvinylidene chloride, low and high density polyethyleneand ethylene-vinylacetate copolymers. Housed within the liquidimpervious outer envelope is a powder-like mix comprising silica 6, asspecified supra., and a pcm material 8. The medical wrap 2 may alsocomprise fastener means such as "Velcro" strips (not shown) to providefor attachment of the wrap around the desired anatomical body part. Inthis case, since outer envelope 4 is liquid impervious, it is notnecessary to utilize the hydrophobic version of the ultra-fine silicaparticles. Additionally, for such medical wrap applications, it isdesirable that the pcm have a melting and freezing point within therange of 0°-60° C.

With respect to FIG. 2A, it can be seen that the silica/pcm combinationprovides a single phase mixture wherein the silica particles 36 arecoated with the pcm 38 material.

FIG. 3 depicts tree wrap 20 wrapped around the base of citrus tree 22.The outer envelope of the tree wrap is similar to envelope 4 shown inFIGS. 1 and 2. The silica-pcm conformable free-flowing powder-like mixcomprising silica particles and pcm is mixed and encased within theenvelope in the same manner as shown for the medical wrap in FIGS. 1 and2. The melting and freezing point of the tree wrap pcm should be fromabout 0°-20° C.

Instead of using a wrap, it is possible to mix the silica-pcmpowder-like composite with the soil surrounding plants, bushes, flowers,etc., to provide warming heat energy protection thereto. In this case,the melting point and freezing point of the pcm should ideally be in therange of about 0°-25° C.

FIGS. 4 and 5 depict a tableware item, a dinner serving tray 30 of thetype used by airlines, etc., that incorporates the free-flowingpowder-like pcm material disposed therein. Here, serving tray 30,comprises a plastic housing 34 that is filled with the silica 36 pcm 38mix. The tray 30 comprises a plurality of compartments 36a-d to act asreceptacles for food and a beverage container. For this particularend-use, a melting point and freezing point for the pcm should be chosenin the range of about 20°-80° C.

Turning now to FIGS. 6 and 7 of the drawings, a textile garment, here avest 100 is shown that is provided with a plurality of pouch portions102 that may be sewn in the garment or attached thereto by otherconventional means. In each pouch 102 is provided a polymeric, liquidimpervious enclosure 104 that is filled with the ultrafine silicaparticles 106 and pcm material 108. In such manner, a pcm containingtextile garment is provided wherein the pcm is chosen so as to releaseheat to the wearer at a desired temperature, usually from 0°-60° C.

FIGS. 8 and 9 illustrate application of the invention to a blanket orquilt 110. Again, pouches 112 formed between the interstices of thelayers of the fabric sheet are lined with a polymeric liquid imperviousenclosure 114 that is filled with the pcm 108 and ultrafine silica 106.The pcm can be chosen so as to liberate heat at desired temperatures,normally from 0°-60° C.

In accordance with the above, it is apparent that the ultrafine particlesize silica provides a convenient carrier for the pcm. The fact that thepcm-silica powder mix readily conforms to different shapes is of benefitwhen the mix is used as a medical hot or cold pack. The dry blendminimizes pcm leakage problems that may otherwise occur. Use ofhydrophobic silica in combination with non-polar PCM provides solutionto a phase separation problem otherwise encountered in high humidityenvironments.

The silica-pcm mix may also be enhanced by the use of an antioxidant inthe formulation. Typically, the antioxidant will be needed only when thecrystalline alkyl hydrocarbon pcm or a polar organic pcm such asethylene glycol, polyethyleneglycol or glycerine is employed. Theantioxidants should be added, when used, in an amount of from 0-1%(weight) based on the weight of the pcm. Exemplary antioxidants includethe well-know hindered phenol materials and aromatic amines. Preferredantioxidants include BHA (butylated hydroxy anisole), Santowhitecyrstals (i.e., 4,4'-thiobis(6-tert-butyl-m-cresol)) and Santowhitepowder (i.e., 4,4'-isopropylidene bis(6-tert-butyl-m-cresol). TheSantowhite products are available from Monsanto.

It should be noted that the polar pcms such as the hydrates, clathrates,semi-clathrates, gas clathrates, water, glycerine, and polyethyleneglycol will not properly wet hydrophobic silicas and for that purposeshould be used only with the hydrophilic silicas.

At present, the silica-pcm composition preferred for use includes ahydrophobic silica and alkyl hydrocarbon pcm wherein the pcm is presentin an amount of 50-80 wt. % of the silica/pcm mix.

It is noted that the ultra-fine hydrophilic silica may also be used withwater and used above the melting point-freezing point (0° C.) so thatthe water can supply both its latent and sensible heat characteristicsto the desired object.

It is to be understood that thermal insulation materials such aspolyurethane or polystyrene foam are desirably used to surround theshrouded pcm/silica composites in order to minimize undesirable heatloss or gain from the environment. For example, insulating materialswould be used when the pcm/silica composites are used as medical wraps,tree wraps, tableware products, garments and blankets and the like. Ineach case, a layer of such thermal insulation would be provided so as tominimize heat loss or gain as the case may be.

In accordance with the patent statutes, the best mode of practicing theinvention has been set forth. However, it will be apparent to thoseskilled in the art that many other modifications can be made withoutdeparting from the invention herein disclosed and described, the scopeof the invention being limited only by the scope of the attached claims.

I claim:
 1. A composition comprising, in combination, a mixture of aphase change material and finely divided silica particles, said silicaparticles having particle sizes of about 7×10⁻³ to about 7×10⁻² microns,said phase change material comprising a crystalline alkyl hydrocarbonhaving a carbon chain length of C₁₄ and greater and being present in anamount of 80% or less by weight and said silica particles being presentin an amount of 20% or more by weight based on the total weight of phasechange material and silica particles in said mixture.
 2. The compositionas recited in claim 1 wherein said composition is in the form of afreely flowing powder-like mixture.
 3. The composition as recited inclaim 2 wherein said phase change material has a melting and freezingpoint of from about -20° to 140° C.
 4. The composition as recited inclaim 3 wherein said phase change material is present in an amount of50-80 wt. %.
 5. The composition as recited in claim 1 further includinga polar compound present in said mixture in an amount of 5-25 wt. %based on the total weight of said composition.
 6. The composition asrecited in claim 1 further including a cementitious material into whichsaid mixture is dispersed.
 7. The composition as recited in claim 6wherein said silica particles comprise a hydrophobic silica.